Treatment of sulfur-containing hydrocarbon mixtures



Aug. 16, 1949. M. M. HOLM ET AL 2,479,238

TREATMENT OF SULFUR'CONTATNING HYDROCARBON MIXTURES Filed Dec. 18, 1945 2 Sheets-Sheet l wow/m ilws'oys'y 0 #5: RAF/7M4 r5 MED/UM STRIPPl/VG N VE N TOIQS Me/v/n M f/o/m Z /oyo FBr'ooke Be/naPaEBer/en bee/7 latented Aug. 16, 1949 TREATMENT OF SULFUR-CONTAINING HYDROCARBON MIXTURES Melvin M. Holm, San Francisco, Lloyd F. Brooke, Berkeley Highland Terrace, and Bernard E. Berlenbach, San Francisco, Calif., assignors to California Research Corporation, San Francisco, Calif., a corporation of Delaware Application December 18, 1945, Serial No. 635,756

17 Claims. 1

The present invention relates to the treatment of sulfur-containing hydrocarbon mixtures, especially petroleum distillates, and pertains more particularly to the refining of petroleum hydrocarbon mixtures containin appreciable amounts of aromatic or unsaturated compounds and sulfur compounds.

In the past the refining of petroleum for removal or separation of relatively large amounts of aromatic and sulfur compounds to yield a product composed substantially of paraflln and naphthene type hydrocarbons has been carried out in various ways which have left much to be desired. Thus, the use of strong sulfuric acid by itself or as a subsequent treatment after extraction with liquid sulfur dioxide, leads to many undesirable residual impurities and reaction products as well as a sulfuric acid sludge disposal problem.

In co-pending application Serial No. 609,676, now Patent No. 2,440,258, of which the present application is a continuation-in-part, there is disclosed and claimed a process of extracting raffinates from sulfur dioxide treatment with hydrofluoric acid, whereby these difficulties in refining are overcome and higher yields of finished products of premium quality are obtained. According to said process, a petroleum oil distillate is first treated with liquid sulfur dioxide at reduced temperatures and in suflicient amount to extract a major portion of the undesirable components (i. e., especially the aromatic and unsaturated compounds along with a portion of the sulfur compounds, etc.) After removal of sulfur dioxide from the raflinate thus obtained, the sulfur compounds remaining, including those introduced by the sulfur dioxide treatment, are substantially completely removed by extraction with hydrofiuoric acid containing not more than about twenty per cent water by weight of the mixture.

In the process of said co-pending application, small but significant amounts of oxygen are usually absorbed by the partially refined raffinate from the sulfur dioxide treatment, such as when this raflinate is sent to intermediate storage, or

sulfur dioxide treated raiilnate, causing difficulty in the treating stage as well as contamination of the finished product.

It is therefore an object of the present invention to provide an improved process of treating sulfur-containing hydrocarbon mixtures with hydrofluoric acid for removal of sulfur-containing compounds, wherein said hydrofluoric acid treatment is carried out in the substantial absence of oxygen.

It is a special object of this invention to provide an improved system for treating sulfurcontaining hydrocarbon oils with hydrofluoric acid in the absence of oxygen, and including an efficient spent hydrofluoric acid regeneration system whereby oxygen is prevented from entering the system via the hydrofluoric acid recycle stream.

It is another object of this invention to provide an improved process of treating hydrocarbon mixtures for removal of aromatic compounds by liquid sulfur dioxide extraction and subsequent removal of sulfur compounds with liquid hydrofiuoric acid, wherein the rafiinate from sulfur dioxide extraction is treated to remove oxygen substantially completely prior to contact with hydrofluoric acid.

Other objects and advantages, including those residing in special combinations of method steps and conditions, will be readily apparent from the following description of preferred embodiments of the present invention taken in reference to the drawings, wherein:

Figure 1 is a schematic fiow diagram of one preferred embodiment of the present invention;

Figure 2 is a schematic flow diagram'of another preferred embodiment of this invention.

According to the present invention, the sulfurcontaining hydrocarbon oil being charged to a hydrofluoric acid treater or reactor is subjected to suitable conditions or treatments for substantially complete removal of oxygen. The oxygen is preferably removed immediately prior to introduction into the hydrofluoric acid treater or the oxygen-free oil is maintained under suitable conditions whereby oxygen is excluded therefrom to avoid further absorption of oxygen. In other words, treatment of hydrocarbon oils containing sulfur compounds with a treating agent comprising a major portion of hydrofluoric acid is carried out in the absence of oxygen. It is found most important to remove oxygen from the charge oil, although it is preferable and usually essential to remove oxygen from both the charge oil and hydrofluoric acid treating agent.

In a liquid-liquid extraction process using liquid hydrofluoric acid as the solvent in the absence of oxygen for the removal of organic sulfur compounds from hydrocarbon oils containing them, the usual conditions may be employed. For example, suitable temperatures at which the hydro- .and the viscosity of the stock being extracted, but for most stocks and the usual pressures and temperatures employed in\ the extraction an amount of hydrofluoric acid between about three per cent and two hundred per cent by volume of the oil stock is generally satisfactory.

It is found that in the absence of oxygen no deleterious action, particularly the formation and deposition of elemental sulfur, is found to occur in the hydrofluoric acid treatment, granted that no other deleterious conditions are present. However, when oxygen, even in small amounts, is permitted to be present, i. e., the teachings of the present invention are not followed, it is found that on contacting sulfur-containing hydrocarbon oils with hydrofluoric acid, noxious amounts of elemental sulfur are formed, deposit throughout the treating system and contaminate the product.

According to the present invention, any suitable means may be employed to remove the small but significant amounts of oxygen in the charge oil as well as from the treating agent containing a major portion of hydrofluoric acid. Thus, stripping, flash or fractional distillation under subatmospheric, atmospheric, or superatmospheric pressures, or other means, may be employed. The hydrofluoric acid may, for example, be redistilled, taking a middle fraction or heart cut.

In one preferred embodiment, oxygen is removed by stripping with an oxygen-free gaseous medium. Suitable stripping agents include gaseous or low-boiling hydrocarbons, such as propane, butane, pentane, etc., or mixtures thereof, such as a pentane fraction, or nitrogen, carbon dioxide, other inert gases, etc. Generally, hydrocarbons of low reactivity are most desirable, and normal parafllns are usually preferred, although in some cases isoparaflins may be employed. Usually condensable hydrocarbons, i. e., normally liquid, relatively low-boiling hydrocarbons, are employed as gaseous stripping medium. It is especially advantageous to employ a relatively large amount of stripping medium, depending upon the nature of the oil and ripping'medium and the amount of oxygen present. Ordinarily, about four to one hundred volumes or more and preferably about ten to forty volumes of gaseous stripping medium per volume of oil are used. The stripping medium is passed through the oil, most desirably when the oil is heated. Generally, the temperature is maintained below about 200 F. and relatively short stripping periods are used in order to avoid color changes in the oil, although higher temperatures up to the boiling point of the oil may be used when exposure of the oil to high temperatures is not disadvantageous. Preferably, stripping is carried out under conditions of reflux for eflicient separation of oxygen from the oil.

Generally, at least part of the hydrocarbon stripping medium becomes dissolved in the oil being treated and ordinarily is removed near the end of the processing, for example, as a final step in obtaining a finished oil. The stripping medium so recovered may be recycled after purification if desired.

In another preferred embodiment, oxygen along with minute amounts of water, which would tend to accumulate and dilute the hydrofluoric acid,

pheric pressure, 1. e., vacuum distillation drying,

whereby oxygen, water, and other impurities are distilled off. A sufllciently low pressure is generally used to avoid detrimental effects, such as color change, etc., from exposing the oil to high tfigmpertures, especially with lamp oils and the l e.

When the hydrofluoric acid treatment is carried out in standard vessels of iron and other metals having reactive surfaces, particularly with reducing tendencies, such as iron, nickel, copper, etc., it is also necessary to remove residual sulfur dioxide as may occur in raflinate oils from sulfur dioxide extraction in order to avoid the formation of elemental sulfur on treatment with hydrofluoric acid. However, when the hydrofluoric acid treatment is carried out in the absence of such metals, 1. e., in a reactor suitably lined with, or formed of, inertmaterials, such as plastics, e. g., Saran, Vinylite, Polythene etc. or metals with inactive surfaces, it is necessary to remove only oxygen completely. Under these conditions it is found possible and sometimes highly advantageous to treat petroleum and like fractions or components thereof with a liquid mixture of sulfur dioxide and hydrofluoric acid to remove simultaneously both aromatic and suZIJr-containing compounds by extraction in the absence of chemical reaction. The same separation can also be obtained with a liquid mixture of hydrofluoric acid and boron trifluoride.

According to the present invention, it is found that improved results without, or at least with diminished, detrimental side eifects, are obtained in the treatment with hydrofluoric acid of hydrocarbon materials containing sulfur compounds, especially oils containing sulfur compounds in appreciable but small amounts, such as above about 0.001% to above 0.01% by weight, when such treatment is carried out in the absence of oxygen. Thus the present process is advantageous in hydrofluoric acid treatment particularly of sulfur-containing petroleum fractions such as naphthas, kerosenes or lamp oils, distillate fuels, gas oils, cracking stocks from asphaltic base crudes, cracked hydrocarbon distillate materials, either normally gaseous or liquid and the like. The invention is especially applicable in the treatment of substantially saturated hydrocarbons containing small but excessive amounts of sulfur compounds for the production of acceptable lamp oils and distillate fuels.

With the present improvement of carrying out the hydrofluoric acid treatment in the absence of oxygen, the hydrofluoric acid treatment may be performed in any suitable manner, and employed together with hydrofluoric acid recovery system, rafllnate oil defluorinators, and other auxiliary operations as desired. However, especially advantageous treating systems are described hereinbelow and in the above-mentioned copending application.

Although a preferred aspect of the present invention is in the combination of liquid sulfur dioxide extraction and hydrofluoric acid extraction, it may also be beneficially employed in the hydrofluoric acid treatment of sulfur-containing petroleum and like hydrocarbon fractions in the absence of prior liquid sulfur dioxide extraction.

Also hydrofluoric acid treating of sulfur-containing hydrocarbon oils for alkylation, isomerization, other molecular rearrangements, etc., will be improved by the present process of treating with hydrofluoric acid, either gaseous or liquid, in the absence of oxygen, although. generally to a lesser extent than is obtained in the preferred application of the present invention, namely, in liquid hydrofluoric acid extraction in the absence of appreciable chemical reaction.

It is apparent that the process of the present invention can be carried out either batchwise or continuously, including continuous countercurrent or concurrent flow of treating agents and oil in the treating stage or stages, or combinations of batch and continuous operation. Thorough mixing of the treating agents with the oil may be obtained by conventional contacting means, such as mechanically-driven agitators, counterflow through packed columns, circulating pumps, atomization mixing, etc. Many suitable types of apparatus can be constructed readily by one skilled in the art, but the invention will be now described in reference to preferred process flow illustrated schematically in the drawings, wherein for the sake of simplicity and clarity there have been omitted certain details, such as pumps, cooling means, heat exchangers, reflux drums, pressure release and control valves, settlers, etc.

Referring now to Figure 1, wherein oxygen is removed from the sulfur dioxide raflinate oil by means of a gaseous or low-boiling hydrocarbon, such as n-pen-tane, the charge oil which may, for example, be a kerosene or lamp oil stock containing aromatic and sulfur-containing compounds, is fed through line I into sulfur dioxide extractor 2 where the oil is contacted with liquid sulfur dioxide entering the extractor 2 through line I 2. The oil or raflinate phase, due to its lower density, moves toward the top of the extractor as the sulfur dioxide or extract phase drops to the bottom of the extractor where it is drawn off through line 3 leading to extract evaporator 4. The recovered sulfur dioxide is led back into sulfur dioxide extractor 2 through line l2 and the sulfur dioxide extract oil, composed chiefly of aromatic hydrocarbons, is withdrawn from the extract exaporator via line 6.

The rafiinate phase is withdrawn from the sulfur dioxide extractor 2 through line I and introduced into raifinate evaporator 8, where the major portion of sulfur dioxide is separated from the oil by distillation and recycled through lines I3 and I2 to sulfur dioxide extractor 2. The raffinate oil is passed through line 9 to intermediate storage tank l0.

Raflinate oil containing small but significant amounts of oxygen and which may or may not contain, but usually does contain, residual amounts or remnants of sulfur dioxide is drawn from tank l and passed through line H into railinate stripper 15, wherein oxygen and any sulfur dioxide present is removed by passing an oxygen-free stripping medium, such as for example, n-pentane, through the heated oil. Stripping medium carrying oxygen and, if present, also sulfur dioxide, discharges from rafilnate stripper through line Hi from whence it passes to a suitable recovery system, other utilization, such as heating fuel, etc., or may be burned. Generally, the raflinate stripper is maintained at conditions of temperature and pressure at which the stripping medium is maintained under reflux.

After withdrawal through line I! from raflinate stripper IS the oxygenand sulfur dioxide-free oil is introduced into hydrofluoric acid extractor 20 where it is mixed with liquid hydrofluoric acid being fed in through line 21. A separation into an extract or hydrofluoric acid phase and a ,raflinate phase is obtained, and the extract or hydrofluoric acid phase is withdrawn from ex-- tractor 20 via line 2| and is fed into hydrofluoric acid regenerator In the regenerator 25, the hydrofluoric acid is recovered from the extract oil phase by passing an oxygen-free stripping medium, such as npentane, through the oil from line 26. Relatively large amounts of stripping medium, such as, preferably, two to ten liquid volumes per volume of extract oil, may :be used. Preferably, suflicient heat is supplied in the regenerator to obtain refluxing conditions at least in the upper portion of the regenerator, consistent, however, with a relatively low bottom temperature to avoid or minimize decomposition or evolution of sulfur compounds. The recovered hydrofluoric acid is recycled to hydrofluoric acid extractor 20 through line 21 into which fresh, make-up hydrofluoric acid freed of oxygen and sulfur compounds may suitably be fed to the system through line 28. T hydrofluoric acid extract oil consisting of a high proportion or the bulk of the sulfur-containing compounds, but no elemental sulfur, is withdrawn from regenerator 25 through outlet 30.

The raflinate phase produced in hydrofluoric acid extractor 20 which includes some of the stripping medium introduced into regenerator 25, is withdrawn through line 3| from the top of the extractor 20 and fed to hydrofluoric acid stripper where at least the hydrogen fluoride is separated from the raflinate oil and recycled to hydrofluoric acid extractor 20 through lines 36 and 21. In some cases it may be advisable to take overhead both the hydrogen fluoride and most of the stripping medium by distillation under reflux, for which purpose there is provided a reflux drum 38 and reflux line 33. In such case, the stripping medium may, if desired, be separated from the hydrogen fluoride before the latter is recycled to hydrofluoric acid extractor 20. Generally, however, it is preferable to remove only the hydrogen fluoride from the oil and to return as reflux to stripper 35 the stripping medium coming overhead with the hydrogen fluoride, leaving most of the stripping medium in the raflinate oil for subsequent separation and recycle as will be described hereinbelow in connection with the second preferred embodiment.

The bottoms from the hydrofluoric acid stripper 35 are fed through line 4| into a final stripper or vessel 42 wherein materials unwanted in the finished product are removed. Thus, when stripping medium remains in the bottoms from hydrofluoric acid stripper 35, it may be removed in vessel 42 in any suitable manner. For example, when the charge oil is kerosene stock and most of the hydrocarbon stripping medium is taken overhead from hydrofluoric acid stripper 35, the vessel 42 is operated as a kerosene s ripper wherein the remaining stripping medium is stripped from the treated oil with steam introduced through line 43. The wet hydrocarbon stripping medium (which may be recovered and recycled) passes overhead through line 44, and the finished oil is withdrawn through outlet 45.

According to the above-described process, the exclusion of oxygen from the system permits the efficient production of finished oil uncontaminated with elemental sulfur, either dissolved or as a finely-divided suspension. Further, the product asvaase the treating vessels or the formation of little or' no diflicultly-disposable lay-products.

In a second preferred embodiment, schematically illustrated in Figure 2, charge 011 for hydrofluoric acid extraction'of the sulfur compounds contained therein is subjected to a pretreatment comprising removal of oxygen as well as water, sulfur dioxide, etc., by distillation. Thus, a charge oil, such as a liquid sulfur dioxide-treated railinate, e. g., kerosene, containing sulfur compounds and drawn from the usual intermediate storage is introduced through line 50 into pre-- treater which may comprise a packed plate, or other suitable column. Preferably the charge oil is maintained under refluxing conditions, at low pressure, such as four to fifteen inches of mercury, whereby oxygen, water, and sulfur dioxide along with some oil pass overhead through line 53 to reflux drum 54. A valve-controlled reflux line 55 may be used to return reflux oil to the pretreater 5|. The low pressure may be maintained in any suitable manner, such as by a steam ejector 60 connected through line 6| to the reflux drum 54. Water may be drained from reflux drum 54 through line 62, and the necessary heat may be supplied to the pretreater 5| by any suitable means, such as a reboiler system 05. 1

Under some conditions when the hydrofluoric acid treatment is combined with prior liquid sulfur dioxide treatment, i. e., Edeleanu treatment, it may be advantageous to combine the functions of the sulfur dioxide evaporator in the usual Edeleanu plant with the above pretreater together into a single unit or vessel.

The oil from pretreater 5| now substantially free of oxygen and other deleterious impurities, such as water and sulfur dioxide, passes through line 09 to a suitable hydrofluoric acid extractor 10, wherein a thorough contacting with hydro- 8 and usually substantially all of the hydrogen fluoride is taken overhead through line 00 preferably into a reflux drum 0.1, from whence part may be used as reflux through a valve-controlled or otherwise suitably controlled line 00 and another part is recycled to hydrofluoric acid extractor I0 through lines 00 and II. In some cases, it may be desirable to take overhead from HP stripper 05 most of the stripping medium together with the hydrogen fluoride; however, ac-

' cording to 'the present preferred embodiment, a

fluoric acid entering through line H is obtained. I

The hydrofluoric acid extractor is operated in the absence of oxygen under conditions for eillcient removal of sulfur compounds.

The hydrofluoric acid extract phase discharges from extractor 10 through line I2, preferably through a heating unit I4 to regenerator I5, wherein oxygen-free hydrocarbon stripping medium, such as gaseous or low-boiling parafflnic hydrocarbon, e. g., n-pentane, from line 80 passes through the extract oil, stripping substantially all or at least a major portion of the hydrogen fluoride therefrom. The regenerator 15 is preferably maintained at an elevated temperature, usually under conditions allowing some reflux, and for this purpose, overhead line H may lead, for example, to a reflux drum 82 having a suitably controlled, e. g., valve-controlled, reflux line 83. From reflux drum 82 a portion, usually a major portion, of the hydrogen fluoride together with hydrocarbon stripping medium is recycled through lines 85 and II to the hydrofluoric acid extractor I0. Oxygen-free make-up hydrofluoric acid may be added into the recycle line 85 through conduit 86. The extract oil containing a considerable proportion of the sulfur compounds discharges from regenerator I5 through disposal line 08.

Substantially sulfur-free rafllnate oil containing stripping medium introduced with the hydrofluoric acid from the regenerator discharges from hydrofluoric acid extractor 10 through line 03 to HF stripper 95. Therein, at least most major portion of the stripping medium taken overhead is used as reflux through line 00. In either case, the presence of the low-boiling hydrocarbon in the HF stripper 05 facilitates the removal of hydrogen fluoride from the ramnate phase at relatively low temperatures.

The bottoms from hydrofluoric acid stripper 05, substantially hydrofluoric acid-free oil, leave through line I05 and pass to a suitable defluorinator unit I00, comprising, for example, one or more vessels packed with lime or bauxite at elevated temperatures, whereby any undesirable amounts of fluorine are removed. Although in some cases defluorination is necessary, generally it is not required and then the defluorinator unit can be omitted.

The substantially fluorine-free oil then passes through line I0'I'into a suitable vessel illustrated, for example, as a kerosene stripper II5 having steam injected thereinto through line I", whereby hydrocarbon stripping medium as well as other materials undesired in the flnished product are suitably removed. As shown, light hydrocarbon stripping medium, e. g., n-pentane, is stripped from the oil with steam via line Ill, the finished oil, e. g., product kerosene, passing out through line 0. The wet hydrocarbon .stripping medium leaves the kerosene stripper II5 through line H9 and preferably enters a separator I20, wherein excess steam is vented through line I2I and condensed water is drained through line I22. The hydrocarbon stripping medium passes from separator I20 through line I25 to a drying tower or drier I30, fitted preferably with suitable auxiliaries, such as overhead line I3I, reflux drum I32, drum vent I", water drain I34, and controlled reflux line I35, as well as heating means, e. g., coil I36.

The recovered hydrocarbon stripping medium leaving drier I30 through line I40 for recycle to regenerator I5, preferably through heating unit I45, may be supplemented with make-up through line I50. In case the make-up stripping medium contains oxygen, water, etc., it may be introduced into the system ahead of the drier I30 whereby only purified hydrocarbon stripping medium is introduced into the system.

The above-described embodiment presents a highly efllcient and advantageous system for removal of sulfur compounds without deleterious side reactions, including formation and deposition of elemental sulfur with attending contamination of the product or formation of undesirable by-products of difllcultly-disposable nature. This system also includes easy regeneration of the extraction solvent and recycle streams which form with the regenerator and hydrofluoric acid extractor a highly advantageous interdependent combination or unit.

Illustrating the present invention, a number of samples of hydrocarbon oils were treated with anhydrous hydrofluoric acid at 70 F. after variout pretreatments of the oils and hydrofluoric acid. In some cases the oils were stripped of I 9 oxygen by passing nitrogen through the boiling oil for about fifteen to thirty minutes at a rate sufficient to displace the volume of the oil in one to flve minutes. In some cases, the hydrofluoric acid was stripped with nitrogen at 69 F. for about the same time and rate. In other instances, as noted, the hydrofluoric acid was redistilled and an intermediate or heart cut taken for the treating step. The results-of these tests are given in the 2. An improved process for removing sulfur compounds from sulfur-containing hydrocarbon oils which contain minor amounts of dissolved oxygen and which precipitate elemental sulfur when intimately mixed with hydrofluoric acid, which comprises, substantially removing dissolved oxygen from the oil and thereafter extracting the resultant oxygen-free oil with hydrofluoric acid to separate an extract comprising following table: 10 sulfur compounds, said extraction step being I O2 Removal HF Rafllnate 'fgf Charge Oil Per Cent s in on 8, other on HF Vol. iff- Total 8, than free Presence of HF/Oil Mm Per Cent Perfree sulfur Cent None. o. 21 Considerable. 15 Do.

80: treated Calif. 0. 48 20 None.

Oil A. do l. 13 20 D0. do 0. 20 0. 035 0. 010 Considerable.

Calif. Gas Oil A 0.35 20 None. -do 1. 19 20 Considerable. 8%11 tfieated Calif. Lamp 0. 49 Very slight.

l I 11 do Very slight amount of No N 0.... 0. 055 20 0.012 0.009 Considerable.

ree 12 do Very slightamount of No No 0. 214 20 0. 011 0. 003 D0.

free 8 +0.024% of other 1'! do do No No.. 0.055 2) 0.009 0.004 Do. 14 ZJz-iDimethyIpentane frac- No free 8 Yes. No l. 2 20 Slight.

on. 15 do do No No-. 0.44 20 Considerable. 16 8% toreated Calif. Lamp -..do Yes No 0. 62 20 Do. 11 an dn Yes- No- 0. a2 20 Do. l8 Sega tfieated Calif. Lamp Ng free S+0.85% other No Yes 0.314 23 0.020 0.011 Do.

1 HF redistilled instead of being stripped. 1% water added. 7 I HF washed with mercury.

The tests in the above table illustrate the necessity of removing oxygen from the ofl and hydrofluoric acid, particularly from the 011. For example, it will be noted that in some cases, (i. e., tests 10 and 14) only slight or very slight amounts of sulfur are found in the rafiinate when oxygen is removed from the oil only and not from the hydrofluoric acid; and where a small amount of free sulfur is not objectionable, the removal of oxygen from the acid may be omitted in treating some oils. However, in other instances (see tests 3, 16 and 17) it is essential to remove oxygen from both the acid and oil. Tests 4 and 18 bring out that oxygen removal from the hydrofluoric acid alone is not enough to prevent formation of a considerable amount of free sulfur. In all cases wherein oxygen is removed from both the hydrofluoric acid and the oil (1. e., tests 1, 2, 5, 6, and 8) there is no free sulfur found in the raifinate.

It is to be understood that the above description of preferred embodiments of the present invention and the exemplary tests are by way of illustration only and that various modifications and changes may be made without departing from the spirit of the present invention as defined by the appended claims.

We claim:

1. In the extraction of sulfur-containing hydrocarbon oils with hydrofluoric acid to remove an extract comprising sulfur compounds the method of preventin the formation of free sulfur during the extraction which comprises substantially removing oxygen from the oil prior to the hydrofluoric acid extraction.

characterized by substantial freedom from the formation of elemental sulfur.

3. The process as defined in claim 2 wherein the hydrocarbon oil consists essentially of saturated hydrocarbons.

4. The method of preventing the formation of free sulfur during the hydrofluoric acid extraction of hydrocarbon oils containing sulfur compounds and minor amounts of dissolved oxygen which comprises passing a stripping medium through said oil to remove oxygen therefrom and thereafter extracting the resulting oxygen-free oil with hydrofluoric acid in the absence of oxygen to separate an extract comprising sulfur compounds.

5. The method as defined in claim 4 wherein the stripping medium is a low boiling hydrocarbon in vapor phase.

6. The method as defined in claim 4 wherein the stripping medium is a low boiling normal paraflln in vapor phase.

'7. The method of preventing the formation of free sulfur during the hydrofluoric acid extraction of straight-run gas oil and kerosene fractions containing sulfur compounds and minor amounts of dissolved oxygen which comprises removing dissolved oxygen from the oil and thereafter extracting the oxygen-free oil with hydrofluoric acid in the absence of oxygen to separate an extract comprising sulfur compounds.

8. The method as defined in claim 7 wherein oxygen is removed from the oil by distilling it under a sub-atmospheric pressure.

9. The method as defined in claim 7 wherein the dissolved oxygen is removed from the oil by passing a vaporized low boiling hydrocarbon through the oil to strip the oil of dissolved oxyn. m. The method of separating an extract com prising sulfur compounds from a hydrocarbon oil which contains sulfur compounds and dissolved oxygen and which forms elemental sulfur when intimately mixed with hydrofluoric acid, which comprises, removing dissolved oxygen from said oil and thereafter extracting theresulting oxygen-free oil with a mixture of sulfur dioxide and oxygen-free hydrofluoric acid, in the absence of oxygen to separate an'extract comprising sulfur compounds, said extraction being characterized by substantial freedom from the formation of elemental sulfur.

11. An improved process for separating aromatic and sulfur compounds from a petroleum fraction comprising said aromatic and sulfur compounds together with paramnic compounds, which process comprises the steps of extracting said fraction with liquid sulfur dioxide under con ditions to produce a rafiinate oil composed mainly of parafllnic compounds and a minor amount of sulfur compounds and a first extract oil composed mainly of aromatic compounds and sulfur compounds, simultaneously removing oxygen and residual sulfur dioxide from said raflinate oil, and thereafter extracting the resultant oxygenand sulfur dioxide-free rafiinate oil with liquid hydrofluoric acid in the absence of oxygen under conditions to produce an extracted oil substantially free of sulfur compounds and a second extract oil containing the bulk of the sulfur compounds.

12. An improved process for separating aromatic and organic sulfur compounds from a petroleum fraction comprising said aromatic and sulfur compounds together with parafflnic compounds, which process comprises the steps of extracting said fraction with liquid sulfur dioxide under conditions to produce a 'raffinate oil composed mainly of paraflinic compounds and a minor amount of organic sulfur compounds and a first extract oil composed mainly of aromatic compounds and a minor amount of said organic sulfur compounds, passing a gaseous normallyliquid low-boiling hydrocarbon through such raffinate oil, whereby oxygen and any residual sulfur dioxide are removed therefrom, and thereafter extracting the resultant oxygenand sulfur dioxide-free raflinate oil with liquid hydrofluoric acid in the absence of oxygen under conditions to produce an extracted oil substantially free of organic sulfur compounds and a second extract oil containing the bulk of said organic sulfur compounds.

13. An improved process for treating a hydrocarbon mixture comprising aromatic and paraffinic hydrocarbons together with organic sulfur compounds, which comprises the steps of extracting said mixture with liquid sulfur dioxide under conditions to produce a ramnate oil composed mainly of parafiinic compounds and a minor amount of sulfur compounds and a first extract oil composed mainly of aromatic compounds and sulfur compounds, passing a gaseous normallyliquid low-boiling normal paraffin through the said raflinate oil, whereby oxygen and sulfur dioxide are removed therefrom, and thereafter extractingthe resultant oxygenand sulfur dioxidefree raflinate oil with liquid hydrofluoric acid in the absence of oxygen under conditions to produce an extracted oil substantially free of sulfur compounds and a second extract oil composed mainly of sulfur compounds.

14. A process of purifying hydrocarbon oils. which comprises treating the same with liquid sulfur dioxide, dissolving the major portion of the constituents to be separated therein, separating the two liquid phases, recovering the hydrocarbon oil from the rafiinate phase, removing from the raflinate oil substantially all dissolved sulfur dioxide and oxygen, treating the partially refined sulfur dioxide and oxygen-free rafflnate oil with liquid hydrofluoric acid, dissolving the major portion of the residual constituents to be separated therein, separating the two liquid phases and separating the highly purified hydrocarbon oil from the hydrofluoric acid in the raffinate phase.

15. A process of purifying hydrocarbon oils, which comprises treating the same with liquid sulfur dioxide, dissolving the major portion of the constituents to be separated therein, separating the two liquid phases, recovering the hydrocarbon oil from the raiiinate phase, removing from the raffinate oil substantially all dissolved sulfur dioxide and oxygen, treating the partially refined sulfur dioxide and oxygen-free rafllnate oil with liquid hydrofluoric acid previously freed of oxygen and sulfur compounds, dissolving the major portion of the residual constituents to be separated therein, separating the two liquid phases, and separating the highly purified hydrocarbon oil from the hydrofluoric acid in the raflinate phase.

16. In a process for the extraction of sulfur compounds from sulfur-compound-containing hydrocarbon oils wherein hydrofluoric acid is contacted with said oils to selectively dissolve sulfur compounds and wherein the selective solvent action of the hydrofluoric acid is attended by concurrent undesired formation of elemental sulfur, the improved method which comprises substantially removing dissolved oxygen from the oil and from the hydrofluoric acid, thereafter extracting the oxygen-free oil with the oxygenfree acid in the absence of oxygen whereby the formation of said undesired elemental sulfur is substantially prevented, separating an extract phase and a rafllnate phase, stripping the hydrogen fluoride from the extract phase with a relatively low-boiling hydrocarbon stripping medium to produce a mixture of hydrogen fluoride and said stripping medium, recycling said mixture to the hydrofluoric acid extraction step, removing hydrogen fluoride from the raflinate phase, which includes said stripping medium, removing said hydrocarbon stripping medium from the resultant hydrogen fluoride-free raffinate, and recycling at least a portion of said hydrocarbon szripping medium to said extract phase stripping s ep.

17. In a process for the extraction of sulfur compounds from sulfur-compound-containing hydrocarbon oils wherein hydrofluoric acid is contacted with said oils to selectively dissolve sulfur compounds and wherein the selective solvent action of the hydrofluoric acid is attended by concurrent undesired formation of elemental sulfur, the improved method which comprises substantially removing dissolved oxygen from the oil and from the hydrofluoric acid, thereafter extracting the oxygen-free oil with the oxygenfree acid in the absence of oxygen whereby the formation of said undesired elemental sulfur is substantially prevented, separating an extract phase and a raflinate phase, stripping the hydro- 13 gen fluoride from the extract phase with a relatively low-boiling hydrocarbon stripping medium to produce a mixture of hydrogen fluoride and said stripping medium, recycling said mixture to the hydrofluoride acid extraction step, removing hydrogen fluoride from the rafiinate phase which includes said stripping medium, removing said hydrocarbon stripping medium from the resultant hydrogen fluoride-free raflinate, removing accumulated extraneous materials from said recovered hydrocarbon stripping medium, and recycling at least a portion of said hydrocarbon stripping medium to said extract phase stripping 14 REFERENCES CITED The following references are of record in the file of this patent:

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